ABSTRACT Over 70,000 adolescents and young adults (AYAs; age 15-39 years) are diagnosed with cancer every year in the United States, more than 8 times the number of younger patients. Survivors of AYA cancer are unique due to their life stage and lack of appropriate services for this population (e.g., fertility preservation, insurance coverage, and educational/vocational support). In contrast to young adult survivors of childhood cancer, little systematic research has been conducted among survivors of AYA cancer to understand their health-related outcomes. At diagnosis and during treatment, AYAs face delayed and limited access to appropriate care, lower clinical trial enrollment, greater treatment-related toxicity and morbidity, and inferior survival. Early post- treatment studies of AYA cancer survivors indicate numerous financial, social, and personal burdens associated with cancer diagnosis and treatment. However, the long-term cancer survivorship experience among survivors of AYA cancer is largely unexplored. Cancer during young adulthood likely leads to loss or delay in achieving milestones of adulthood (e.g., full-time employment, financial health, marriage, having children) following cancer treatment. Additionally, young adult Hispanic cancer survivors are understudied, despite evidence that Hispanics experience significant differences in health-related outcomes. Project Milestones will address these research gaps by recruiting and surveying a population-based cohort (n=2,000, ~42% Hispanic) representing long-term (3-10 years post-diagnosis) survivors of the most prevalent cancers found in older AYAs diagnosed between ages 21-39. Our specific aims are: (1) To assess risk and protective factors of the achievement of milestones of adulthood and to compare achievement of milestones between survivors of AYA cancer and non-cancer comparators; (2) To assess risk and protective factors of long-term well-being and health related quality of life (HRQOL); and, (3) To assess risk and protective factors of engagement in cancer-related follow-up care. For all outcomes, we will examine age, cancer type, and ethnic differences. Among Hispanic survivors, we will examine cultural factors associated with study outcomes. This population-based study will include prototypical AYA cancer cases ascertained through the Los Angeles County Cancer Surveillance Program Cancer Registry. Self-report surveys will assess milestones of adulthood, well-being, HRQOL, and cancer-related follow-up care. Multivariable regression analyses will identify protective and risk factors for milestones, HRQOL and cancer-related follow-up care. Comparative analyses will identify differences between Hispanic and non-Hispanic AYA survivors and between AYA cases and non-cancer comparator participants (from the California Health Interview Survey). Results will inform the development of guidelines and programs supporting survivors of AYA cancer, including addressing and reducing health disparities affecting those of Hispanic ethnicity.